Power transmission is generally carried out by overhead power lines or underground cables which are generally confined to the metropolitan area. The overhead high voltage power lines located in or near a construction site are rather vulnerable to incidents in which the overhead high voltage power lines are damaged or hit by a crane in motion. Such an incident as described above may be fatal.
As shown in FIG. 1, a typical power line network consists of a voltage build-up transformer T1 for changing a power generating plant (G) voltage level of 13.8 kV or 20 kV to an ultrahigh voltage of 161 kV, which is then transmitted to the ultrahigh voltage load users via overhead power lines W1 supported by the overhead power line towers. In the meantime, the ultrahigh voltage power is transmitted to a primary power substation H1 in which the ultrahigh voltage level of 161 kV is reduced to a level of 69 kV for use by the special high voltage load users. The power is further transmitted via overhead power lines W2 to a secondary power substation H2 in which the power voltage is further lowered to a voltage level of 11.4 kV. The high voltage of 11.4 kV is distributed via distribution lines D1 to the high voltage users. In the meantime, the high voltage of 11.4 kV is changed by a transformer T2 (11.4 kV/110-220-380V), which is mounted on an overhead power line pole, to the lower voltage for household use.
It is very likely that a crane in motion may accidentally hit high voltage power lines located in or near the construction site. It is therefore conceivable that such an accident can be averted by an alarm device designed to warn of danger of hitting the overhead high voltage power lines by the crane in motion. The Japanese Industrial Safety Association disclosed an alarm device which is designed on the basis of the induced current of a grounded suspension arm or metal sphere. The induced current can be easily affected by the grounding situation of the crane. In addition, the induced portion and the connection line are rather lengthy such that they can be easily affected by the magnetic coupling.